A photo of PS4 page from the Now in Stock website shows that the standalone systems, as well as various bundles, have sold out at all major retailers (except for the auction site, eBay). Some shops reportedly ran out of stock on or right before Christmas while others were exhausted of units on the following day.
On the other hand, the Xbox One appears to be readily available. Retailers have both the standalone console and various bundles in stock. You can compare the two stock situations from the Now in Stock website, as well as some hardware images of the PS4 and Xbox One, in the slideshow to the left of this article.
One possible explanation for the disparities is that Microsoft may be shipping more units in the United Stared when compared to Sony. Moreover, the PS4 may be in higher demand due to it being the lower-priced next-generation that is also reportedly more powerful.
Sony and Microsoft released their respective new consoles last month in North America. The PS4 came out first in the middle of the month while the Xbox One launched one week later on Nov. 22.